Area Forecast Discussion
Issued by NWS Anchorage, AK

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330
FXAK68 PAFC 242358
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
358 PM AKDT Sat Jun 24 2017

.ANALYSIS AND UPPER LEVELS...
A broad upper level trough stretching from the Gulf to just east
of the Alcan border is beginning to be the focal point for
showers and thunderstorm development from the Copper River Basin
northward into the eastern Interior. This low is also bringing
enough northerly flow into Cook Inlet to scour out the remaining
stratus that lingered over the Anchorage Bowl and Matanuska Valley
overnight. A narrow ridge of high pressure that had been centered
over the eastern Bering has now shifted east over the Southwest
Mainland. This has allowed a marine layer of low clouds and fog
to reach much further inland where it has persisted into the
early afternoon. This ridge is beginning to break down under the
influence of both an upper level trough to the north and an
approaching upper level low encompassing much of the central-
western Bering. This low brought rain and gale force winds today
which are beginning to spread into the Dutch Harbor and Pribilof
Island areas.

&&

.MODEL DISCUSSION...
The models are all in very good agreement through Monday and have
initialized well today. In particular, they are all in generally
good agreement in the placement of convection over the Copper
River Basin and Talkeetna Mountains today, which matches well
where development can be seen on early afternoon satellite
imagery. One typical problem spot from the models that is posing
issues to the forecast is tracking the depth and location of the
marine layer that has spread inland from the southwest coast.

&&

.AVIATION...
PANC...VFR conditions and light winds will continue into Sunday.
Showers over the Talkeetna Mountains this afternoon will drift
southward across the Mat Valley and front range of the Chugach
this evening. However, all showers are expected to remain east
of the terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
An upper level low situated over the northern Gulf of Alaska is
currently pushing a weak easterly wave across the Copper River
Basin which has already helped produce several thunderstorms.
Additional heating this afternoon and early evening will allow
for these thunderstorms to increase in coverage and move into the
Matanuska and Susitna Valleys. Much of Southcentral Alaska will
continue to see clearing skies as weak high pressure aloft moves
into the region.

Sunday looks to be quite a nice day across much of the region
although it may start off a little cloudy during the morning
across Anchorage and the Kenai Peninsula. Ample heating on
Sunday will allow for thunderstorm development across the normal
mountainous terrain including the Chugach Mountains. Storm motion
will be from west to east so all shower and thunderstorm activity
will remain out of Anchorage proper. Cloud cover will increase on
Monday which will stabilize the atmosphere and likely prevent
thunderstorm development as temperatures top out 5 to 8 degrees
cooler on Monday than on Sunday.

Tuesday will see a pattern shift as a low pressure system moves
into the western Gulf of Alaska and a frontal boundary out ahead
of it brings rainfall to the Gulf coast. Gusty winds are expected
to develop on Tuesday across Turnagain Arm, the Copper River Basin
and Knik River Valley in response to the frontal orientation. This
may also keep inland locations fairly dry as downslope flow dries
out the lowest levels.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Weak shortwave ridging this afternoon has helped stabilize the
atmosphere sufficiently to suppress thunderstorm development.
Evening showers across the interior mountains will weaken
overnight leaving mostly dry conditions. Onshore flow this
evening, especially across Bristol Bay, will allow for the
redevelopment of low stratus and fog overnight, and it may become
more dense given the slackening winds and building surface high
pressure across eastern Bristol Bay.

Sunday will be mostly dry everywhere, but a few isolated to
scattered showers can`t be ruled out across the Alaska Range.
Otherwise, the main change in the pattern comes Sunday evening and
overnight as a Bering warm front moves onshore. This will bring
gusty southeast winds along the coast as well as light rain as the
front moves inland Monday. The front stalls through Monday night
before the upper level trough moves inland, reinforcing the stalled
front and helping support more widespread rain and showers. Expect
cool and wet conditions through Wednesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
The upper trough over the Western Bering has now emerged with the
North Pacific shortwave and associated warm front. Satellite
imagery depicts rather impressive (for summer) upper level jet
left-exit region divergence underneath broad warm air advection.
This will spread rainfall eastward from the Pribilof Islands and
Unalaska to the coast of Southwest Alaska by Sunday evening. Winds
from the south/southeast will also reach gale force tonight and
Sunday before weakening. As the front reaches the coast, expect an
extensive dry slot behind the front to move through, thus ending
rain and bringing in widespread low stratus along with westerly
winds. The upper low will move east through Tuesday, bringing
chances for showers for the whole Bering Sea and Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The extended forecast beginning next Wednesday brings increased
chances for rain across the southern mainland and Gulf as energy
from the Pacific swings north over the mainland. The main upper
level system centers over the Bering mid week and then a stronger
vort max rotates through the flow off Kamchatka. This vort max
develops a new progressive low center that takes an easterly track
across the central and eastern Aleutians during the end of next
week. The ECMWF and Canadian models are in better agreement with
continuous weak energy moving from the Gulf to Southcentral ahead
of the newly developing low center. This supports increased cloud
cover and higher rain chances across the southern mainland and
surrounding waters through the end of next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 155 170 172 174.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...MC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...KH



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